The number of people infected by Ebola in the three countries worst affected by the outbreak has passed 20,000, with more than 7,842 deaths in the epidemic so far, the World Health Organization said on Monday.

Cumulative case numbers in Sierra Leone, Liberia and Guinea stood at 20,081, the WHO said in a statement. More than a third are laboratory-confirmed cases in Sierra Leone, which has become the worst-hit country in the worst outbreak of the disease on record.